DO THE TON

Blood Sweat Tears and Grease => Projects => Cafe Racers => Topic started by: JadusMotorcycleParts on Aug 20, 2017, 13:58:39

Title: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Aug 20, 2017, 13:58:39
The title says it all.  This is going to be more of a speed and performance focused build than the last.  Less time and energy spent on finishes, more time spent on finessing and fettling.  I am hoping to implement and share a lot of ideas that both myself and other awesome members of this forum came up with through the process of the previous build.  The stock SR will get about 120kph, all Jadus bikes - both the test mules and the black standard get around 135kph.  All make around 20hp at the crank.  So looking to get 30hp with some trick mods to be able to get to 160kph or... the ton!

Something I noticed when I completed the black SR was that it was the nicest bike I have ever build, and I didn't/don't like it.  Not because I don't love it, but because I hate having nice things.  I hate the feeling of being 'precious' with something - not wanting to scratch the paint, not wanting it to get dirty etc.  That is why I will never own a nice car.  I want to use, thrash and trash my toys.  I don't want to spend time cleaning and polishing!  What a waste of time!  So this bike will be more along those lines and I can't wait to get stuck in.

I thought I would be ready to move onto the next model of bike to design parts for by now, but I feel I have one more good SR250 in me before that.  Who knows, maybe even more parts for the SR250 along the way!

The reason I want to document the build here is twofold.  One, I can share what I am doing with a greater audience, and two, it opens things up for a great discussion for ideas and allows me to get feedback from more experienced members of the forum who have 'been around the block' a few times  ;)

You can also follow the hashtag on insta:  #100mphSR250

Let it begin!  ;D
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Aug 20, 2017, 14:07:23
I have been holding off on starting this thread - waiting to make absolutely sure everything was tied up with the black bike.  But I started the thinking process long ago and have already got stuck in with a couple basics - engine out of the frame plus some angle iron supports in the frame.  This will allow me to run two projects in parallel - an engine project and a chassis improvement project.  The SR uses the engine as a stressed member, so messing with the frame (sitting on it, getting ride heights, foot peg positions etc) without extra supports is not a good idea - it would bend under load.

I have read a lot since I began building bikes as well.  All the books below I have read cover to cover and have many book marks.  Some of the mods I decided to do might sound strange to some, but I have pondered long and hard and have read and re-read about these mods to make sure.  I have also purchased a great chassis design pdf - more on that later.

Some of the number goals might be a stretch but I am absolutely confident I can get 160kph.  Will it be a first for an SR250?  Maybe not - if you know of anyone doing it, please point me in their direction  :D  100kg, possibly, 110kg, definitely.  30hp?  At the crank, yes, at the wheel, maybe 26!
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: Ryan Stecken on Aug 20, 2017, 14:20:30
I'm in man!!!cant wait to see thst thing finished!

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Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JMPUK on Aug 20, 2017, 15:02:39
This will be epic!!

Can't wait to see it
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: zap2504 on Aug 22, 2017, 12:34:56
Like your other builds, I'm in! I also have had that Smith book for a very long time. Originally got it when interested in modding car engines.

One thing to suggest to your list of engine mods is thinking about pumping losses. There was an article by Dave Searle in Motorcycle Consumer News about this a couple months ago. Thumpers are notorious here because they don't have other cylinders working together to offset the crankcase pumping action. Many builders just vent their crankcases to look "cool" but it really doesn't do anything to decrease pumping loss (just keeps oily junk out of the carb like an oil catch can would); I got a Krank Vent off evilbay to experiment with creating a partial vacuum in the crankcase instead of trying to get all that air in/out of that skinny hose outlet (still planning). Krank Vents are a 1-way check valve and seem to be very popular with the H-D crowd as they reduce crankcase pressure and keep gaskets from blowing. My thoughts are that the engine will pump through the Krank Vent a couple times until the partial vacuum is created, then no more pumping.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: firebane on Aug 22, 2017, 16:53:46
This will be interesting. I had a SR250 that topped out about 110kph but vibrated like hell lol
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: Hurco550 on Aug 22, 2017, 17:01:11
I enjoyed watching you last one, so of course im going to watch this one too =)
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: Sav0r on Aug 22, 2017, 17:39:53
I shed around 38kg from my RD350. It has a battery, a stock charging system, a heavily filled stock tank, all turn signals and lights. 30kg should be in reach without too extreme an effort for the SR. The wheels, forks brakes, fenders, seat, and a bunch of the other junk are pretty easily eliminated or replaced with modern bits. 
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Aug 23, 2017, 17:24:05
Thanks for the support!

Like your other builds, I'm in! I also have had that Smith book for a very long time. Originally got it when interested in modding car engines.

One thing to suggest to your list of engine mods is thinking about pumping losses. There was an article by Dave Searle in Motorcycle Consumer News about this a couple months ago. Thumpers are notorious here because they don't have other cylinders working together to offset the crankcase pumping action. Many builders just vent their crankcases to look "cool" but it really doesn't do anything to decrease pumping loss (just keeps oily junk out of the carb like an oil catch can would); I got a Krank Vent off evilbay to experiment with creating a partial vacuum in the crankcase instead of trying to get all that air in/out of that skinny hose outlet (still planning). Krank Vents are a 1-way check valve and seem to be very popular with the H-D crowd as they reduce crankcase pressure and keep gaskets from blowing. My thoughts are that the engine will pump through the Krank Vent a couple times until the partial vacuum is created, then no more pumping.

Yepp, that is for sure on my list.  Someone (maybe even you?) posted some good stuff on the other build thread from the LS650 Savage forum about a system.  I'll see If I can dig that up.  It is also covered in these books :)
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Aug 23, 2017, 17:25:07
I shed around 38kg from my RD350. It has a battery, a stock charging system, a heavily filled stock tank, all turn signals and lights. 30kg should be in reach without too extreme an effort for the SR. The wheels, forks brakes, fenders, seat, and a bunch of the other junk are pretty easily eliminated or replaced with modern bits.

True!  Thanks for the encouragement.  I'll start with the big obvious things then see what level I need to go into to shed the last few kgs.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: Sav0r on Aug 23, 2017, 17:31:21
Hit me up if you have questions. I have a few simple but effective tricks beyond the common ones to get my RD down to the that weight. I didn't even have to spend that much extra dough, and in many cases the results were a vast improvement, the weight savings were just a bonus.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Aug 24, 2017, 18:16:49
Hit me up if you have questions. I have a few simple but effective tricks beyond the common ones to get my RD down to the that weight. I didn't even have to spend that much extra dough, and in many cases the results were a vast improvement, the weight savings were just a bonus.

Sweet, I'll do that!
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Aug 24, 2017, 18:19:29
Managed to 'build' a 'rustic' table for filming some more install and tech vids.  Then found some time to blast the engine as well.  This time a pretty rough job compared to the last engine - running with the more time on performance things than looks!  I actually tried soda this time too, but it just wasn't abrasive enough to remove the clear coat on the engine cases or the oxidisation. 

Oh yeah, then put on some old 3D printed prototypes to dress it up a little and put it on the stand I made a last year.  Never thought it would be so handy!
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: datadavid on Aug 25, 2017, 07:17:37
Is it even physically possible to get 30hp out of these? Staying tuned..
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: canyoncarver on Aug 25, 2017, 12:24:59
I'm in. 
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: Sav0r on Aug 28, 2017, 17:57:30
Is it even physically possible to get 30hp out of these? Staying tuned..

Physically possible, absolutely. Technically possible, maybe.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: Alex jb on Aug 29, 2017, 09:24:31
Physically possible, absolutely. Technically possible, maybe.
Isn't it the other way round?

Either way we want to see you try :)


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Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: CrabsAndCylinders on Aug 29, 2017, 14:03:55
Subscribed!  :)
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: Sav0r on Aug 29, 2017, 14:23:41
Isn't it the other way round?

Either way we want to see you try :)


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My point is that the potential exists, but does the technical know how in this specific case exist? Maybe, maybe not.

I'm not OP though, so I'm not building it.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: Alex jb on Aug 29, 2017, 16:34:22
Ok, gotcha.
I think so too.
I buying a Honda 250 twin motor to tear down make some mods and if it works out, heading for the ton too.


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Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: jpmobius on Aug 29, 2017, 17:56:14
Aerodynamics will be of critical importance with this much power to attain 100 mph.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Aug 29, 2017, 18:15:12
Is it even physically possible to get 30hp out of these? Staying tuned..

Dunno man, but will sure as hell try.  From this research I have done, I will be going down the path of many small improvements to make a well tuned whole, rather than one or two drastic changes.  I'll be working a lot on lightening the entire valve train - with the intention of coping with more cam and lifting the rpm ceiling 1000rpm so it pulls to 10,000.  Will increase cc's and compression as well.  More will be revolved along the way!
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Aug 29, 2017, 18:17:12
Aerodynamics will be of critical importance with this much power to attain 100 mph.

Absolutely.  This will actually be more crucial than weight reduction and outright power.

That being said, I would like to try with minimal aero stuff.  At this early stage, I am hoping a good body position (clip-ons, rear sets, tank hugging) plus a small headlight faring (to throw a little wind over my helmet) might do.  But it will require testing.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Aug 30, 2017, 18:09:04
I think I shared this resource on the other thread but here it is again:  http://www.klemmvintage.com/bighorntech.htm

Yes it is a 350cc 2 stoke, but the chassis - suspension and brake set ups are very interesting I have taken a lot of inspiration from what they have done.  I will be doing similar stuff  :)

I also purchased Tony Foales Handling and Chassis Design pdf.  It is super dense.  And a pretty hard read I thought.  I have picked up a few key points but otherwise, will not be going into extreme details like in there.  https://www.tonyfoale.com

I do know what rake and trail I would like and what ride height/eye to eye rear suspension I would like though.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Aug 30, 2017, 18:11:41
I did some quick research and it seems like 30hp should get 160kph.  Some pretty nifty 250cc Ducati singles have done it!  http://www.motorcyclespecs.co.za/model/Classic%20Racers/moto_morini_250_gran_premio_1964.htm

Given, that particular model had a fairing, BUT could reach 140mph...
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Sep 10, 2017, 06:11:57
I'll be doing a kickstart conversion on this bike too.  But many people just add it and have it as a compliment to the electric start.  I will be removing not only the starter, but also the many internal parts necessary to make it all work.  Some of these are even rotating mass = power sappers. 

I did a little exercise where I collected all the parts from the two different assemblies (from a spare engine I have) and weighed the two set ups.  The total weight savings will be around 2.5kg.  And that is just in the engine!  Then if I were to include the fact that the battery will be able to be half the size as well, there will be a few hundred grams extra savings there too.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: Alex jb on Sep 10, 2017, 10:33:52
Thorough work!

If you are re-wiring then you could dump the e-start wiring and swap the hand control for one without button, it all helps!




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Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Sep 11, 2017, 01:26:37
Thorough work!

If you are re-wiring then you could dump the e-start wiring and swap the hand control for one without button, it all helps!


Good point!
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: datadavid on Sep 11, 2017, 07:21:26
Dont install the kicker, start it with a roller!
Title: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: Alex jb on Sep 11, 2017, 11:51:37
Dont install the kicker, start it with a roller!
That's a man in a van following you to Starbucks to re-start you after your mochachocachino?! 


Joking aside, old Skool privateers used to bump start their bikes to dump the kick start shaft didn't they?


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Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Sep 11, 2017, 17:12:18
Haha, yes, considered having it bump start only actually.  But with previous experience bump starting a high comp thumper (on my own), no thanks.  I'll keep some small practicalities on the bike  ;D
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: CrabsAndCylinders on Sep 11, 2017, 17:58:43
Dont install the kicker, start it with a roller!

I did that on a couple bikes, it was fun and cool, I thought, except when stalling going up a steep hill during rush hour.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Sep 17, 2017, 10:44:52
It may sound like a boring start, and it isn't where a lot of other people start but perhaps should...  Tyre choice.

For this lightweight racer I will matching tyres to rims first.  Then re-lacing them to the stock hubs with the Jadus 18" stainless spoke sets.

The tyre of choice is the Avon road rider, for many reasons.  It is tried and true, can handle the speed and handles really well - from both reviews and personal experience with one up front on the other Jadus SR.

http://www.avon-tyres.co.uk/motorcycle/roadrider

The size of choice will be a 90/90-18 up front and a 100/90-18 at the rear - will be plenty wide enough for this kind of power and will keep things light (always good with less rotating mass).

This means that rim choice will be aluminium for light weight and a size 2.15"-18 rim for the front, 2.50"-18 for the rear.  Here are some guides I used:
https://ridewrightwheels.com/pages/motorcycle-tire-wheel-fitment-chart
https://www.scribd.com/doc/253007466/Motorcycle-Rim-Width-Tire-Size-Chart
http://dropbears.com/motorcycles/utilities/tyrerim.htm

So next pay check, wheels will be the first purchase. 
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: stroker crazy on Sep 17, 2017, 18:51:51
If you haven't already chosen the rims, you could try Morad TC:
http://www.central-wheel.co.uk/rims/morad_rim/morad_rim.html (http://www.central-wheel.co.uk/rims/morad_rim/morad_rim.html)

Crazy
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Sep 18, 2017, 17:22:54
If you haven't already chosen the rims, you could try Morad TC:
http://www.central-wheel.co.uk/rims/morad_rim/morad_rim.html (http://www.central-wheel.co.uk/rims/morad_rim/morad_rim.html)

Crazy

Good advice, have used their services many times and been very satisfied.  But this time I will go with a Swedish supplier who manufacture their own spokes and are very helpful - and also local!  They are a reseller for Excel rims too, so will go with them this time.  Will report if any major difference in quality to the Morad's.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Sep 22, 2017, 03:31:31
Figured I may as well introduce some of the planned engine mods (at this stage - can always change).  So far no plans to do much with the bottom end, it'll all be improving gas flow, capacity and compression.

_ Custom made high comp 77mm piston (263cc, retains stock sleeve, no case boring required) - piston already measured up and sent to Special Piston Services in Australia - they specialise in one offs and a re super friendly with a lot of experience:  http://www.specialpistonservices.com  will try raise cr from 8.9:1 to 9.5:1 or even 10:1 if we can work out a good dome.

_  Hot cam, the most aggressive profile, regrind by Tighecams in Australia: http://tighecams.com.au

_ Ported head to allow better airflow - or, to allow the cam to do its thing

_ Custom titanium intake valve, SS exhaust valve, both with narrower stems and both much lighter than stock - will maintain same valve size though (more on that later)

_ Valve seats custom made from NBC to wick away heat from the ti valve better

_ Bronze valve guides - also for better heat transfer to the head

_ Single beehive valve springs

_ Titanium keepers, seats and retainers

_ Lightened rockers and tappets

Then I'll be doing a few things on the intake and exhaust side too - as mentioned previously some kind of crank case ventilation scavenging system and probably a harmonic intake.

As an experiment, I already tried lightening some rockers I had from a spare engine.  Good and worth while result.  Especially becuase the weight shaved is at the points furthest from the fulcrum - where they 'weigh' the most to the valve train. Those small tabs at the end of the rocker are for the decomp lever on the XT and TT model engines and because the rocker is a symmetrical part, Yamaha have used it on the intake side as well - where it is not needed (where neither are needed in the case of the electric start SR).  I will be converting to kick yes, but am hoping with the right technique, won't need the decomp set up from the XT engine.

Hoping to lift the rpm ceiling to 10,000 by putting the whole valve train on a diet with the aforementioned mods  ;D
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: killerx on Sep 26, 2017, 16:09:41
You should be able to drill some holes in the webbing of the rockers.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Sep 27, 2017, 03:13:54
You should be able to drill some holes in the webbing of the rockers.

Thanks man, this is the kinda advice/encouragment I was looking for  ;D  I am so sure I saw a great post about someone doing just that over on a Yamaha quad forum, but can't seem to find it now.  The only gains through this method though would be smaller holes right out towards the ends of the rocker.  Because drilling holes close to the fulcrum may make it lighter, but won't do anything for lightening the load on the cam or the valve spring.  Worth investigating on these scrap rockers though for sure!
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: jpmobius on Sep 27, 2017, 11:01:41
Reducing weight is almost always a good thing as long as strength is not compromised.  However, it is worth keeping in mind your end goal.  What do you achieve?  Do you have in mind increasing safe operating rpm?  Generally higher revs make possible higher output.  Or do you plan on more aggressive cam profiles which will increase valve train accelerations? (same consequences valve train wise as increased rpm) Valve trains are often the limiting problem in output because the natural frequency of the springs needed to control the valve train components becomes too close to the engine rpm.  Drop a spring on the floor and it will "ring" or oscillate for a bit at its natural frequency.  If you "excite" it by driving it to this natural rate (or a harmonic of the natural rate) it will try to continue this rate of oscillation instead of following the camshafts intent.  Commonly called valve float and can be very destructive.  The solution is to require a lighter spring (or stiffer)(lighter meaning lower in mass in this context) that has a higher natural frequency so as not to be excited by the available rpm.  A lighter spring  will of course require lighter valve train parts.  Valves, pushrods, lifters, spring retainers, etc all move (mostly) linearly so simply making them lighter overall works toward this goal.  Rocker arms on the other hand rotate, so how you lighten them is less straight forward.  Mass at the center of rotation is of little concern as it moves very little, where mass at the end of the arm(s) is very important.  However, loads on the arm(s) generally become higher with stiffer springs and/or increased rpm, so one should be careful when lightening these parts so as not to compromise the beam strength of the arm.  Likely you would prefer a stronger rocker than a lighter rocker - which is not to say you still don't want the lightest you can get!  Anyway, the point is to be careful and thoughtful when altering your parts.  I'd say the lightening you have done so far has no down side, but potentially reducing the beam strength will provide negligible gains at best and potentially unnecessary troubles.  Having your valve train disassemble itself at max revs can be unpleasant!
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Sep 27, 2017, 14:08:47
Thanks for the inout jp!  I'll refer you to Reply #34 in this thread - just a few posts up on this page.  There I state all the engine mods I plan on doing with all the goals and some of the reasons.  Yes, lift the rpm ceiling to 10,000 rpm by putting the whole valve train on a diet (hence the whole rocker arm exercise) and using modern beehive valve springs - to avoid many of the problems you mention. 

Good point about the rocker mods though, this was my gut feeling...  Further mods might risk strength or may even result in flex at high rpm?
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Oct 09, 2017, 14:45:24
Over the weekend I managed to start the tear down of the top end and do a bit of a measure up.  I wanted to know the exact weight of the piston assembly so we have a baseline for the new big bore piston.  I hope that with new methods, we can achieve a bigger piston with the same weight as this one so that the balance of the engine (both the primary balance for the crankshaft and the secondary from the balancer shaft) remains the same - so not to effect the vibration levels.  If this cannot be achieved, we will taper bore the piston wrist pin to get the weight back to stock.

Then the piston producer wanted to know valve angle and wanted me to mark the valve centres on the piston so he has a reference for machining out the valve clearance pockets.  The valve angles in the photos don't show up very accurate because of the perspective, but they were both 115 degrees.

I also measured the compressed height of a used head gasket and the deck height.  Deck height seems to be zero...  So just relies on valve to piston clearance and the clearance from the head gasket height.  Eventually I will cc the head and add this to the volume of the compressed gasket.  Then I will be able to get an exact compression ratio and be able to calculate and predict a new compression ratio with the new piston and the new swept volume.

The head volume is listed in the manual as 30.4cm3, head gasket thickness as 1mm (which is correct for a compressed one) and compression ratio as 8.9:1 (for the 239cc versions) and I have no reason not to believe this, but I want to measure it anyway.  I will also use that compression ratio and these listed numbers to reverse calculate the head and gasket volume just to double check my numbers and see if there are any differences.
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Oct 18, 2017, 17:32:29
Tonight I managed to cc both the head and the piston dome.  These methods are pretty common practice but are usually done in a much more controlled, accurate way with proper equipment!  This was sort of a hack job to see if Yamaha were correct in the manual, where they state that the head volume is 30.4cc - which I didn't really understand, it just didn't add up in my calculations.  But then, after all the measuring and math, it turns out that the stated figure is actually total volume at TDC - taking into account the head volume, the gasket volume and the piston dome volume!

My rough measurements (all are written on the calculations sheet) all added to a volume at TDC of 31.7cc which is 1.3cc off what the manual says.  I was therefor happy to say that Yamaha are most likely correct and I am a little off haha, so will use this figure.  But good to know I was in the ball park. 

I calculated backwards for the different engine releases as well to double check the numbers - which added up.

With this number, I was able to calculate a new, predicted CR with the 77mm piston...  Which will roughly be 9.6:1 for a 9% increase over stock.  Is that enough?  Would 10:1 be better?  Imma look into it a little more.

In any case, I have many options to increase comp a little.  But the two most practical would be a slightly thinner solid copper gasket (will need a custom copper gasket anyway for the custom piston dims, so may as well be a little thinner) or to shave a little off the head.  Just by reducing compressed head gasket volume or actual head volume 2cc, the CR will be up to 10:1  ;D
Title: Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
Post by: JadusMotorcycleParts on Oct 18, 2017, 17:37:25
Here is a great post by a guy who did this process properly and accurately:  http://skrunkwerks.com/skrunk/measuring-compression-ratio-method-1/

Nicely documented too!

Just a little background perhaps for those CR numbers...  The North American SR was in fact a proper 250cc bike - displacing 249cc.  So with a TDC volume of 30.4 it gave a CR of 9.19, or as the specs round to, 9.2   Then all the other 239cc SR models have a CR of 8.8618, or as rounded to in the specs, 8.9  ;)